Heretofore, as a typical photosensitive substance to be mixed in photosensitive coating layers of photosensitive lithographic printing plate and photosensitive screen printing plate, a condensate (so-called diazo resin) of diphenylamine-4-diazonium salt (or 4-phenylamino-benzenediazonium salt) and formaldehyde has been used. Production methods for such a multifunctional diazo condensate are described in specifications of U.S. Pat. Nos. 2,679,498, 2,922,715, 2,946,683, 3,050,502, 3,311,605, 3,163,633, 3,406,159, and 3,277,074. These diazo condensates, although having many advantageous properties as practical photosensitive substances, have problems which have yet to be solved.
A first problem is that these diazo condensates are relatively less soluble in organic solvents. This is especially true when inorganic ions such as halide ions or phosphate ion are used as counterions to the diazonium group of these diazo compounds, and, when sulfonate ions such as benzenesulfonic acid, toluenesulfonic acid, or an alkylsulfonic acid are used, their solubility is insufficient in such organic solvents as glycol ethers, alcohols, ketones and the like. Such low solubility in organic solvents is a problem with these diazo condensates when used as photosensitive agents in photosensitive lithographic printing plates.
Various investigations have been attempted to solve such a problem. For example, Japanese Patent Publication 53-24449/1978 discloses the use of a special compound such as 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid as a counterion to the diazonium group, which enhances the solubility in organic solvents.
Japanese Patent Publication 49-45323/1974 discloses the use of a special compound such as 4,4'-bismethoxymethyldiphenylether as a condensation agent for diphenylamine-4diazonium salt in place of formaldehyde, which provides the same improvement. Furthermore, Japanese Patent Publication Laid-open 2-3049/1990 discloses the use of hexamethoxymethyl-melamine as a special condensation agent.
A second problem is that, since diazo condensate using diphenylamine-4-diazonium salt as a basic structural unit has an absorption in the visible light region at 420-500 nm, it tends to absorb light of this wavelength and decompose, and is thus difficult to be handled under visible light free from ultraviolet, which constitutes a disadvantage for production and use.
As a method which simultaneously solves the second and first problems, there has been investigated a novel multifunctional diazo compound which basically differs in chemical structure from that of prior art diazo compounds. The thus obtained novel diazo compound comprises 4-dialkylaminobenzene-diazonium salt as a basic structural unit, in place of the conventional diphenylamine-4-diazonium salt.
For example, Japanese Patent Publication Laid-open 54-30121/1979 and Japanese Patent Publication Laid-open 61-273538/1986 use two or more 4-dialkylaminobenzene-diazonium salts, which are ester bonded to form a multifunctional diazonium salt. Further, Japanese Patent Publication 157332/1989 uses 4-dialkylaminobenzene-diazonium salt, which is bonded in the form of a polyalkylamine to form a multifunctional diazonium salt. However, although these novel types of diazo compounds have advantageous properties, their production is generally not easy. Furthermore, diazo compounds which are made multifunctional by ester bonding in the above laid-open Japanese Patent Publications have a problem in that the ester bond tends to be hydrolyzed.
A third problem is that diazo compounds used for photosensitive screen printing plates are essentially required to have a high solubility in water. In this respect, no diazo compounds have been reported which have the performance over the prior art diazo compounds comprising diphenylamine-4-diazonium salt condensed with formaldehyde. In Japanese Patent Publication Laid-open 2-11198/1990, diphenylamine-4-diazonium salt is condensed with formaldehyde to obtain highly water-soluble multifunctional diazo compounds. However, these compounds have been developed to improve the developability of the lithographic printing plate after exposure with an alkaline aqueous solution, but do not provide an improvement for screen printing.
A fourth problem is that currently used condensates of diphenylamine-4-diazonium salt with formaldehyde are insufficient in photosensitivity. Therefore, photosensitive compositions using the current multifunctional diazo compounds as photosensitive agents are difficult to shorten the practical exposure time, and, when the exposure time is reduced, the image after development tends to be colored in brown.
As described above, the prior art compounds have problems which have yet to be solved. Including a primary object of the present invention, these problems are summarized as follows:
1) Development of a multifunctional diazo compound which can be handled under visible light free from ultraviolet.
2) Development of a multifunctional diazo compound which, as a photosensitive agent for a lithographic printing plate, is high in solubility in appropriate organic solvents, and is easy to be developed with a developing solution mainly comprising an alkaline aqueous solution.
3) Development of a multifunctional diazo compound which is suitable as a photosensitive agent for screen printing plate.
4) Development of a multifunctional diazo compound which is high in photosensitivity.
5) Development of a multifunctional diazo compound which is easy to be synthesized, and establishment of a low-cost production method for the compound.
6) Development of a photosensitive composition which effectively utilizes the properties of the thus obtained novel multifunctional diazo compound(s).